1. Field of the Invention
The present invention relates to an organic electroluminescence device, and more particularly, to an organic electroluminescence device which can be easily fabricated and used as a light emitting device for various displays.
2. Description of the Prior Art
Although many trials for fabricating organic electroluminescence devices (hereinafter, referred to as an EL device) have been reported from a view point of the elucidation of the state of electrons of the material thereof, these device have low luminance because a carrier is difficult to inject into a light emitting layer composed of an organic substance (hereinafter referred to as light emitting layer) from an electrode. To solve this problem, Tang et al. made a two layer structure composed of a hole transport layer of an organic substance laminated on an organic emitting layer to realize an EL device with high efficiency and high luminance (U.S. Pat. No. 4,539,507). Thereafter, there are made a device comprising a three layer structure having a light emitting layer disposed between an organic electron transport layer and a hole transport layer of an organic substance (hereinafter, these are collectively referred to as charge transport layers) [Japanese Journal of Applied Physics, 27, page 269 (1988)] and EL devices having light emitting layers which are doped with various dyes to give various colors [Journal of Applied Physics, vol. 65, page 3610 (1989)].
For these organic EL devices reported up to now, the light emitting layer and the charge transport layers were made by vapor depositing an organic substance in vacuum. A vapor deposition method, however, is not suitable for mass-production and has a limit in the production of a device having a large area. Further, when the EL device is used as a back light for illuminating a non-light emitting display such as an LCD and the like, a request to make an area of the EL device larger is increased and the EL device needs to be made on a large scale. Incidentally, when low molecular weight organic fluorescent materials such as tris (8-hydroxyquinoline) aluminum, anthracene, etc., which have been used up to now, are used as the light emitting layer, these materials are difficult to be made to a thin film by coating when they are independently used without matrix materials.
On the other hand, there is a trial to make a light emitting layer by spin coating a polymer semiconductor such as polyvinylcarbazole in which a fluorescent substance such as perylene, triphenylbutadiene, etc. is dispersed [Polymer, 24, 748 (1983)], but this trial has a problem because of low luminance. Therefore, a method for making a thin a film for highly luminant EL devices could not help being restricted to limited methods such as a vapor deposition method, etc.
The organic EL device made by the vapor deposition has a problem in that it has lower light emitting luminance and light emitting efficiency per electric power than other light emitting devices. To solve this problem, it is needed that an efficiency of charge injection from an electrode is improved to realize a high luminance with a low drive voltage. Further, when the EL device acts as a display, a light must be uniformly emitted over a large area. An organic thin film layer constituting the organic EL device has a very thin film thickness of about 1000 angstroms, and thus a problem is also pointed in that a light is liable to be irregularly emitted which is supposed to be caused by an irregular substrate and an uneven electrode.